1. Field of the Invention
The present invention relates to a high efficiency inverter circuit capable of delivering sinusoidal voltage at high frequency, and particularly to an emergency lighting system including a tuned inverter for operating at least one gaseous discharge lamp.
2. Description of the Prior Art
Electric power failures due to inclement weather conditions and equipment breakdowns have been a plague for many years. Recently, widespread areas have suffered blackouts due to overloading of the generating or transmission equipment. A power failure, no matter what may be the cause, may very well jeopardize human life and therefore there are many installations which require some type of emergency lighting system that will automatically come into operation upon the occurrence of a power failure. The high efficiency of a fluorescent lamp makes it especially valuable for use in an emergency lighting system.
Many of the emergency lighting systems available on the market utilize a rechargeable battery as the source of power for the system. Since there is a finite limit on the length of time that a battery can power an illumination system, it is rather important that the system have a high efficiency. Presently available systems are generally of the type using a transistor inverter. In a typical arrangement, a single lamp or group of lamps is used for both the normal AC operation of the lighting system and for the emergency system, using a battery as the power source for energizing the transistor inverter when the AC line voltage fails. A principle limitation of such inverter systems is relatively low efficiency. This low efficiency requires the use of a larger and hence more expensive battery to achieve acceptable operating time during emergency conditions.
Prior art circuits for sensing AC line voltage in emergency lighting systems are varied. One such system has a full wave responding inhibit voltage developed from a separate winding than that used to charge the battery, the inhibit winding being more tightly coupled to the primary than is the battery charge winding. With such an arrangement, the influence of variable battery voltage is attenuated. Another system appears to use a low reactance transformer with a dropping resistor to regulate the charging current to the battery, the resistor thus preventing the battery from clamping the transformer winding and influencing the AC inhibit voltage.
It is desirable therefore to provide an emergency lighting system capable of maximizing the operating time on a given battery charge.
In accordance with the present invention, there is provided, in an emergency lighting system having a high reactance transformer with a secondary winding and a primary winding adapted for coupling to an AC energy source, circuit means for monitoring the AC source voltage. Included are means for coupling the secondary winding of the transformer with a non-linear load during one half cycle of the AC source voltage and means for coupling the transformer secondary winding with a linear load during an alternate half cycle to provide DC voltage proportional to the AC source voltage.